Method and apparatus for validating coins

ABSTRACT

A device for validating a coin comprises an electro-magnetic sensor, means for monitoring a first signal generated by the sensor and means for deriving a measurement from a second signal generated by the sensor. The event of the first signal taking a predetermined threshold value is used to derive a measurement from the second signal.

FIELD OF THE INVENTION

The invention relates to a method and apparatus for validating coins.

BACKGROUND OF THE INVENTION

The invention is intended especially for use in validating coins havingan inner, central core made of a first metallic material and an outerring made of a second metallic material. Such coins are commonly knownas bi-colour coins. The invention is also useful for coins having two ormore outer rings of different compositions. One or more of the core andouter ring(s) may be formed of layers of two or more materials, in a“clad” construction.

The term coin is used throughout the specification to mean any coin(whether genuine or counterfeit), token, slug, washer, or other metallicobject or item, and especially any metallic object or item which couldbe used in an attempt to operate a coin-operated device or system. A“valid coin” is considered to be an authentic coin, token, or the like,of an acceptable denomination and which a coin-operated device or systemis intended selectively to receive and to treat as an item of value, andespecially an authentic coin of a monetary system or systems in which orwith which a coin-operated device or system is intended to operate.

Various techniques for validating coins and, in particular, for testingthe material of coins, are known. Coin testing apparatus is well knownin which a coin is subjected to a test by passing it through apassageway in which it enters an oscillating magnetic field produced byan induct-or and measuring the degree of interaction between the coinand the field, the resulting measurement being dependent upon one ormore characteristics of the coin and being compared with a referencevalue, or each of a set of reference values, corresponding to themeasurement obtained from one or more denominations of acceptable coins.It is most is usual to apply more than one such test, the respectivetests being responsive to respective different coin characteristics, andto judge the tested coin acceptable only if all the test results areappropriate to a single, acceptable, denomination of coin. An example ofsuch apparatus is described in GB-A-2 093 620.

More specifically, it is known from EP 0 710 933 to test bi-colour coinsusing an inductive sensor, in the form of pair of coils, in combinationwith two optical sensors. in the apparatus described in EP 0 710 933 theoptical sensors are used to control the operation of the inductivesensor to produce a first reading of the coin when the coin is centredon the coils and a second reading when the outer rim portion of the coinis centred on the coils, that is, when the rim portion in combinationwith other adjacent portions of the coin are in the field of thesensors.

A disadvantage of the device mentioned above is that, if an opticalsensor becomes dirty, the accuracy of the timing of the reading of theinductive sensors, which is controlled by the optical sensors, may bereduced. Further, the optical sensor may fail to operate altogether if,for example, the light source or detector is blocked by a piece of dust.Another disadvantage is that the device uses a measurement taken whenboth the outer rim material and the centre material of the coin, andthus the interface between the two materials, are within the field ofthe coils for validating the coin. It has been found that the effect onan inductive sensor of a portion of a bi-colour coin including theinterface between the two materials changes over the life of a coin, andalso it will not necessarily be the same for all coins of the same type,so that coin validation based on a measurement taken over the interfacemay not be accurate. All the above disadvantages can lead to a validcoin being rejected or an invalid coin accepted.

SUMMARY OF THE INVENTION

The object of the present invention is to mitigate or overcome one ormore of the above-mentioned disadvantages.

The present invention provides a device for validating a coin comprisingan electro-magnetic sensor, means for deriving first and second signalsfrom the sensor and means for deriving a measurement from the secondsignal, wherein the event of the first signal taking a predeterminedthreshold value is used to derive said measurement.

The second signal is representative of the material of a coin passingthrough the sensor and the first signal can be considered as a triggerwhich is used to select the appropriate part of the second signal.Because a signal from the electromagnetic sensor itself is used as atrigger, there is no need for external timing triggering means like, forexample, the optical sensors in the prior art. Thus, the disadvantagesencountered with the optical sensors are eliminated. Also, the deviceoperates with fewer components, which can reduce the cost.

The threshold value can be chosen to trigger measurement for any desiredpoint on a coin. Preferably, the threshold value is chosen to derive ameasurement for a non-central portion of a valid coin.

The invention is suitable for validating coins having a central core andmore than one outer ring, for example, bi-colour coins.

The first and second signals may be sampled at intervals. Interpolationtechniques may be used to derive a measurement from the second signal.

Preferably, the sensor comprises a pair of coils connected in aself-excited oscillator circuit, the coils being arranged opposite eachother on either side of a path for a coin. The first signal mayrepresent the oscillator frequency and the second signal the oscillatoramplitude. Alternatively, the first signal may represent the oscillatoramplitude and the second signal the oscillator frequency.

Preferably, the threshold value is selected to derive a measurement foran outer ring portion of a valid coin. In the case of a bi-colour coin,a measurement is preferably obtained for only the outer ring portion ofthe coin, that is a measurement obtained when only the outer ringportion of the coin influences the sensor. By deriving a measurementwhen only the outer rim portion of the coin is next to the sensor, thedevice avoids the difficulties encountered when taking a “mixedmeasurement”, that is a measurement of both materials of the coin at thesame time including the interface.

Preferably, the measurement is taken as the coin moves downstream fromthe sensor, that is, when the centre of the coin has sassed the centreof the sensor, where the motion of the coin is more stable.

The invention also provides a device for validating a bi-colour coin,wherein the first signal is used to derive a measurement representativeof only the outer rim material of a valid coin.

The invention further provides a method for validating a coin comprisingderiving first and second signals from a sensor, detecting the event ofthe first signal taking a predetermined threshold value and using thedetection of that event to derive a measurement from the second signal.

The invention also provides a method for validating a coin comprisingmonitoring a first signal generated by the sensor, and using the firstsignal to derive a measurement from a second signal generated by thesensor, which measurement is predominantly representative of anon-central portion of a valid coin. Preferably, the method is forvalidating a bi-colour coin, wherein the first signal is used to derivea measurement representative of only the outer rim material of a validcoin.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a device for validating coins in accordance with thepresent invention is described below with reference to the accompanyingdrawings, of which:

FIG. 1 is a schematic drawing of a coin-sensing area in a coinvalidating mechanism;

FIG. 2 a is a simplified detail of FIG. 1;

FIG. 2 b is a cross-section taken along the line A—A of FIG. 2 a;

FIG. 3 is a block diagram;

FIG. 4 is a diagram of a coin in a sequence of positions relative to asensor;

FIG. 5 is a graph showing a first waveform obtained from a coin sensor;

FIG. 6 is a graph showing a second waveform obtained from a coin sensor;

FIG. 7 a is a diagram showing a detail of the waveform of FIG. 5;

FIG. 7 b is a diagram showing a detail of the waveform of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows schematically a coin sensing area within a mechanism forvalidating coins. As shown in FIG. 2 a, the sensing area comprisessensors 1, 2, 3 which are used to obtain measurements that arepredominantly dependent on the, coin material, coin thickness and coindiameter respectively (referred to hereinafter as the material sensor,thickness sensor and diameter sensor). The sensors 1, 2, 3 are arrangednext to and extend normal to a ramp 4 which provides a path for a coin(not shown). The thickness sensor 2 and diameter sensor 3 are knownelectro-magnetic inductive sensors, operated in accordance with knowntechniques, and will not be described here in further detail.

As shown in FIG. 2 b, the material sensor 1 is an electromagneticinductive sensor comprising a pair of coil assemblies 5, 6 arrangedopposite each other on either side of the coin ramp 4 and coupledtogether. Each coil assembly 5, 6 is arranged within a respective coilassembly 7, 8 of the thickness sensor 2, as described in EP-A-0 489 041.Each coil assembly comprises a coil and a ferrite. The diameter of eachcoil assembly 5, 6 of the material sensor is approximately llmm, whichis smaller than the diameter of the core of ail well-known bi-colourcoins currently in circulation.

As represented in block diagram form in FIG. 3, the material sensor 1 isconnected to a validation circuit 9 for driving the sensors, processingthe signals from the sensors and determining validity and denomination.The validation circuit 9 includes an oscillator (not shown) connected tothe coils of the coil assemblies 5, 6 of the material sensor 1, which isused to generate a signal from the coils which is representative of thecoin. The circuit 9 also generates suitable output signals including asignal, depending on the outputs from the various sensors 1, 2, 3 forcontrolling the operation of an accept/reject gate 10 within the coinvalidation mechanism.

FIG. 4 shows a bi-colour coin 11 in a sequence of different positionsrelative to the material sensor 1. When any part of a coin is next tothe sensor 1, it influences the inductance and resistance of the coilsin the sensor which in turn affects the frequency and amplitude of theoscillator output. As the coin passes through the field produced by thecoils, the frequency and amplitude in the oscillator output change. Afirst signal, which represents the changing frequency of the signal inthe oscillator, and a second signal, representing the changingamplitude, are generated in the validation circuit 9, and examplewaverforms for those signals are shown in FIG. 5 and FIG. 6. The firstsignal represents a relationship (for example, the difference or theratio) between the frequency of the oscillator at any given time and theidle frequency (that is, the frequency when there is no coin influencingthe sensor) and is known as the “frequency shift”. Similarly, the secondsignal represents a relationship (for example, the difference or ratio)between the actual amplitude of the oscillator output and the idleamplitude and is known as the “amplitude shift”. The sensor is driven atlow frequencies, that is frequencies below about 120 kHz.

As different coins pass through the sensor 1, different frequency andamplitude signals are generated, having waveforms dependent on thecharacteristics of the coin. As described below, for any given coininserted into the validator, the frequency and ampl-tude signals aremonitored and two values, representative of the coin, are derived fromthe amplitude signal and used to test the coin.

The frequency signal is used to derive a measuremen from the secondsignal by using a threshold value as a “trigger”. The threshold value isthe value of the frequency signal when only the outer rim portior of avalid coin is next to the sensor, as determined by calibration, so that,for subsequent valid coins, a measurement is derived for that samepoint, giving a measurement representative of only the outer material.

When a coin is inserted in the validator, the validation circuitmonitors the frequency signal to detect when the signal crosses thatthreshold value. In this example, the signal is monitored to detect whenthe signal crosses the threshold value and is decreasing, that is, for avalid coin, when the coin is at the point C in FIG. 4 so that only thetrailing edge of the coin is next to the sensor. A measurement for thatpoint is then derived from the values of the amplitude signal, asdescribed in more detail below, and that measurement is representativeof only the outer rim material of the coin.

The frequency and amplitude signals are sampled at a constant rate onceevery millisecond, and the sampled values are stored and monitored bythe validation circuit.

A measurement is derived from the sampled amplitude signal using aninterpolation method which will be described with reference to FIGS. 7 aand 7 b which show an approximation of the frequency signal in theregion of the threshold value and the corresponding amplitude signalrespectively. When a sampled value of the frequency signal falls belowthe threshold value (T), that sampled value (f₂), the previous sampledvalue of the frequency signal (f₁) and the corresponding sampled valuesof the amplitude signal (a₂ and a₁) are selected or retrieved from thestore. A value for the amplitude signal a, at the point t_(T) at whichthe frequency signal took the threshold value can be obtained usinginterpolation, in accordance with the equation:$a_{T} = {{\left( {T - f_{2}} \right)\frac{\left( {a_{1} - a_{2}} \right)}{\left( {f_{1} - f_{2}} \right)}} + a_{2}}$

and the value a_(t) so obtained is used to validate the coin, asdescribed below.

The sampling rate is relatively fast having regard to the rate of changeof the frequency signal, so that the approximations are sufficientlyaccurate.

The sampling rates and/or times of sampling of the frequency signal andthe amplitude signal need not be the same. The amplitude signal may, forexample, be sampled asynchronously.

The validation circuit also monitors the amplitude signal to detect whenthe coin is centred on the sensor (point B on FIGS. 4, 5 and 6) andtakes a measurement from the amplitude signal, a_(B), at that point.There are known techniques for detecting when a coin is centred on thesensor, which is indicated by a local maximum in the amplitude signal.The size of the coils of the material sensor is such that the outer rimof a valid coin does not influence the coils when the centres coincide.Thus, a measurement of the amplitude signal at point B is representativeof the centre material of the coin.

In the manner described above, two representative values, a_(T), anda_(B), are obtained from the amplitude signal, which are values for theouter rim material and for the centre material.

The values a_(T) and a_(B) are used to validate the can by comparingthem with stored acceptability data, in the form of “windows”, that is,stored upper and lower limits (see GB 1 452 740). A first window isprovided for the value a_(T) and a second window for the value a_(B).If, for a given coin, each of the values a_(T). and a_(B) falls withinthe respective window (and the measurements from the sensors 2 and 3 arealso deemed acceptable), then the coin is deemed to be valid and thevalidation circuit generates a “coin accept” signal which controls thecoin accept/reject gate.

The apparatus can be adapted to validate a different bi-colour coin byadjusting the stored acceptability data. Such adaptation can be achievedsimply by altering the software used in a control means and does notrequire the hardware to be changed. The apparatus can also be used tovalidate more than one bi-colour coin type, using a different thresholdvalue for each of the coins to be validated, the value obtained at eachthreshold point being compared with a respective window. By usingseveral threshold points to trigger a material measurement, it ispossible to identify where the material of a coin changes, so that, forexample, the width of the outer ring of a bi-colour coin can becalculated.

Various modifications to the device described above are possible.

More particularly, other methods for using the representative values tovalidate the coin could be used. The acceptability data could insteadrepresent a predetermined value such as a median, the measurements thenbeing tested to determine whether or not they lie within predeterminedranges of that value.

Alternatively, the acceptance data could be used to modify eachmeasurement and the test would then involve comparing the modifiedresult with a fixed value or window. Alternatively, the acceptance datacould be a look-up table which is addressed by the measurements, and theoutput of which indicates whether the measurements are suitable for aparticular denomination (see, for example, EP-A-0 480 736 and U.S. Pat.No. 4,951,799).

Instead of having separate acceptance criteria for each test, themeasurements may be combined and the result compared with storedacceptance data (see, for example, GB-A-2 238 152 and GB-A-2 254 949).Alternatively, some of these techniques could be combined, for example,by using the acceptability data as co-efficients (derived, for example,using a neural network technique) for combining the measurements, andpossibly for performing a test on the result.

Alternatively, instead of using two values selected from the amplitudesignal, validation could be performed using the value a_(T) from theamplitude signal and the value of the frequency signal at the point whenthe coin is centred on the coils, which also gives a valuerepresentative of the centre material. Again the values so obtainedcould be used separately or in combination.

In all the above modifications, the roles of the frequency signal andthe amplitude signal could be reversed, so that the amplitude signalfunctions as the trigger and vice versa. Other signals from a sensorinfluenced by a coin could be monitored, for example, the real andimaginary component of the impedance of an inductor, as described inGB-A-2 287 341, or the amplitude and phase shift, as described in GB-A-2244 837.

It is not necessary to use two coils. A sensor comprising only one coil,as described, for example, in GB-A-2 266 399, could be used.

The invention is not limited to use in validating bi-colour coins. Thetechniques and apparatus described can be adapted for deriving ameasurement for any given point on a particular coin, using one or morepredetermined threshold values. Thus, the apparatus could be used, forexample, for taking a measurement of each ring of a coin having two ormore concentric rings of different material, or for validating a coinwith a hole in the middle.

Our co-pending application, GB 9703769.1, entitled “Coin Validator”filed on Feb. 24th 1997, also relates to validating bi-colour coins, andthe contents of that document are incorporated herein by reference.

What is claimed is:
 1. A device for validating a coin comprising anelectro-magnetic sensor, means for deriving first and second signalsfrom the sensor and means for deriving a measurement from the secondsignal, wherein the event of the first signal taking a predeterminedthreshold value is used to derive said measurement.
 2. A device asclaimed in claim 1 wherein the threshold value is selected to derive ameasurement for a non-central portion of a valid coin.
 3. A device asclaimed in claim 1 wherein the first signal is used to select a periodfrom the second signal and a measurement is derived from said selectedperiod.
 4. A device as claimed in claim 1 wherein the first and secondsignals are sampled at intervals and interpolation techniques are usedto derive a measurement from the second signal.
 5. A device as claimedin claim 1 wherein the sensor comprises a coil arranged on one side of apath for a coin.
 6. A device as claimed in claim 5 wherein the sensorcomprises a pair of coils connected in an oscillator circuit, the coilsbeing arranged opposite each other on either side of a path for a coin.7. A device as claimed in claim 6 wherein the first signal representsthe oscillator frequency and the second signal represents the oscillatoramplitude.
 8. A device as claimed in claim claim 6 wherein the firstsignal represents the oscillator amplitude and the second signalrepresents the oscillator frequency.
 9. A device as claimed in claim 1for validating a coin comprising two or more concentric rings of two ormore different materials, wherein the threshold value is selected toderive a measurement for an outer ring portion of a valid coin.
 10. Adevice as claimed in claim 9 for validating a bi-colour coin wherein thethreshold value is selected to derive a measurement for only the outerring portion of a valid coin.
 11. A device as claimed in claim 1 whereinthe measurement is taken as the coin moves downstream of the sensor. 12.A device as claimed in claim 1 adapted to derive a measurement for thecentre of material of a valid coin.
 13. A device as claimed in claim 1comprising a store of acceptance data representative of a bi-colourcoin.
 14. A device as claimed in claim 1 comprising a store of aplurality of threshold values.
 15. A device as claimed in claim 14 forvalidating two or more different types of bi-colour coins.
 16. A deviceas claimed in claim 1 wherein predetermined threshold values are used tomeasure the width of a portion of a coin.
 17. A device for validating acoin comprising an electromagnetic sensor and means for deriving firstand second signals from the sensor, wherein the first signal is used toderive a measurement from the second signal, which measurement ispredominantly representative of a non-central portion of a coin.
 18. Adevice as claimed in claim 17 for validating a bi-colour coin, whereinthe first signal is used to derive a measurement representative of onlythe outer rim material of a valid coin.
 19. A method for validating acoin comprising deriving first and second signals from a sensor,detecting the event of the first signal taking a predetermined thresholdvalue and using the detection of that event to derive a measurement fromthe second signal.
 20. A method for validating a coin comprisingmonitoring a first signal generated by a sensor, and using the firstsignal to derive a measurement from a second signal generated by thesensor, which measurement is predominantly representative of anon-central portion of the coin.